A phosphonitrilic acid ester has a very wide range of application as a plastic and its additive, rubber, fertilizer, medicine, etc. Particularly, derivatives of a phosphonitrilic acid ester oligomer or a phosphonitrilic acid ester polymer have very excellent characteristics such as excellent flame retardancy, higher anti-hydrolysis property and higher heat resistance when compared to conventional phosphate esters. Therefore, such derivatives of a phosphonitrilic acid ester oligomer or a phosphonitrilic acid ester polymer have a very promising application in flame retardant and nonflammable materials with respect to imparting flame retardancy and incombustibility to a plastic by a non halogen flame retardant agent, which in recent years has attracted increased social interest. Furthermore, because a resin composition to which they are added shows a very low dielectric constant, industrialization is strongly desired as a flame retardant agent for use in an electronic material such as a printed circuit board material and a semiconductor sealing material.
Among phosphonitrilic acid esters, cyclic trimers represented by the following general formula (7) and cyclic tetramers represented by the following general formula (8) are particularly attracting attention in recent years.
(wherein Q represents an aryloxy group or an alkoxy group)
(wherein Q represents an aryloxy group or an alkoxy group)
The phosphonitrilic acid ester represented by the following general formula (9) does not contain a chlorine atom (hereinafter referred to as a chloro group) bonded to a phosphorus atom in the structural formula. However, since it is usually prepared by alkoxylation or aryloxylation, there remains a monochloro compound having a chloro group as represented by the following general formula (10) in the products obtained by aryloxylation and/or alkoxylation reaction. It is very difficult to replace all the chloro groups by an aryloxy group and/or an alkoxy group at the time of preparation, and it is particularly difficult to replace one chloro group which remained in the last in a molecule.
(wherein Q represents an aryloxy group or an alkoxy group, and m represents an integer of 3 or more)
(wherein Q represents an aryloxy group or an alkoxy group, and m represents an integer of 3 or more)
The remaining chloro group may be hydrolyzed and generate a hydroxy compound represented by the following general formula (11). This may cause an increase in the acid value of the reaction product, or generate a P—O—P bond by a cross-linking reaction to form a gel, and the excellent characteristics which the phosphonitrilic acid ester has may not be exhibited.
(wherein Q represents an aryloxy group or an alkoxy group, and m represents an integer of 3 or more)
For example, when a phosphonitrilic acid ester in which substitution on the aryloxy group and/or alkoxy group have not completed is added as a flame retardant agent to a resin such as a polyester resin, particularly a polycarbonate resin that is easily decomposed with an acid, the resin itself may be decomposed by phosphate traces derived from P—OH portions contained in the phosphonitrilic acid ester. In connection with this, not only thermal properties such as flame retardancy and heat resistance of the resin composition but various mechanical physical properties will be impaired. Furthermore, in the case of a resin for use in electronic materials, problems such as reduced dielectric performance may occur.
The method for preparing a phosphonitrilic acid ester includes (1) a method in which a a metal chloride or a solvent, if needed (for example, see JP-A-51-21000). This method reduces unreacted chloro groups that remain in a phosphonitrilic acid ester. However, there arises a problem when a glycidyl group in the epoxy compound opens a ring and reacts with the phosphonitrile dichloride, chloro atoms remain in the molecule. Furthermore, there is also a problem that the epoxy compound does not sufficiently react by itself with the phosphonitrile dichloride, and in order to complete the reaction, the amine compound must be used, and the reaction process and operation become cumbersome.
A method is known in which toluene is used as a reaction solvent and a cyclic phosphonitrile dichloride is reacted with an alkaline metal arylate while adding thereto a linear or cyclic nitrogen-containing organic compound to enhance nucleophilic reactivity, thereby lowering the amount of residual chlorine below 0.01 mass % (for example, see JP-A-2001-2691). According to this method, it is possible to reduce the amount of residual chlorine in a phosphonitrilic acid ester. However, a large amount of the nitrogen-containing organic compound is required, which entails a cumbersome operation of collecting the nitrogen-containing organic compound from the reaction product or the solvent and this makes the method disadvantageous for industrial, application.
There is also a known method in which dioxane phosphonitrile dichloride is reacted with an alkaline metal salt of a hydroxy compound, (2) a method in which a hydroxyl compound is reacted with a phosphonitrile dichloride using a tertiary amine as a hydrochloric acid trapping agent, and (3) a method in which a phase-transfer catalyst such as a quaternary ammonium salt is used and a hydroxy compound and a phosphonitrile dichloride are reacted in the presence of a hydrochloric acid trapping agent such as a secondary or tertiary amine.
Among the related art processes for the preparation a phosphonitrilic acid ester, there is commonly known method in which toluene or xylene is used as an inactive solvent for the reaction, and a phosphonitrile dichloride is made to act on an alkaline metal alcoholate or an alkaline metal phenolate prepared by azeotropic dehydration from an alcoholic compound or a phenolic compound with a hydroxide of an alkaline metal thereby preparing a phosphonitrilic acid ester (for example, see U.S. Pat. No. 4,107,108). This method, however, has a problem where it is difficult to replace all the chloro groups in a phosphonitrile dichloride, for example, by a bulky phenoxy group, where the reaction takes a long time and results in a high content of monochloro compounds.
Another method is known in which a phosphonitrile dichloride, an epoxy compound, and an amine compound react together using a catalyst such as is used as a reaction solvent and an amine-type phase-transfer catalyst and a pyridine derivative as an agent for trapping a hydrogen halide generated in the aryloxylation or alkoxylation are added for the reaction (for example, see JP-A-64-87634). However, this method takes a long time for completing the reaction. In addition, although the pyridine derivative used in a large amount is expensive and desirably to be re-used, it changes to a hydrogen halide salt after the reaction and needs a cumbersome regeneration processes such as an alkali treatment and distillation, which is problematic.
There are additional methods in which toluene as a reaction solvent and a quaternary ammonium salt as a phase-transfer catalyst are used (for example, see JP-A-64-87634, JP-A-60-155187). These methods use a quaternary ammonium salt in a large amount and the operation of collecting the quaternary ammonium salt is cumbersome. Moreover, since a lot of water is used at the time of a reaction, the reaction system is a biphasic system of water and an organic solvent, and a phosphonitrile dichloride tends to undergo hydrolysis and the reaction temperature cannot be raised. Therefore, a long time is needed for completion of the reaction. On the other hand, when the reaction temperature is raised in order to enhance the reactivity, the following problems arise such as hydrolysis becomes noticeable, phosphate traces derived from P—OH portions generate and gelling tends to take place by a cross-linking reaction.
There is also a known method in which monochlorobenzene is used as a reaction solvent, and a cyclic phosphonitrile dichloride and an alkaline metal arylate and/or an alkaline metal alcoholate are reacted while the amount of moisture in the reaction system is controlled (for example, see JP-A-2000-198793). This method allows particles of the alkaline metal arylate and the alkaline metal alcoholate in the reaction solvent to be finely dispersed by reducing the amount of moisture at the time of preparing the alkaline metal arylate and the alkaline metal alcoholate, and thereby improves the reactivity. However, such improvement in the reactivity is still insufficient and the reaction time is long until completion.
There is a known method in which an aliphatic hydrocarbon having 6 to 9 carbon atoms is used as a reaction solvent and an alkaline metal alcoholate is prepared from an alkaline metal and an alcohol and then reacted with a phosphonitrile dichloride dissolved in monochlorobenzene (for example, see U.S. Pat. No. 3,939,228). It is possible to complete the reaction in a relatively short reaction time in this reaction. However, an alkaline metal is expensive and difficult to handle since it has very high reactivity with moisture, and therefore it is problematic when applied to industrial uses.
Furthermore, there is a known method in which dichlorobenzene or trichlorobenzene is used as a reaction solvent, and an alkaline metal arylate or an alkaline metal alcoholate are reacted with a phosphonitrile dichloride polymer (for example, see French patent No. 2,700,170). In this method, there is no description about the amount of moisture in the reaction system at the time of an aryloxylation and/or an alkoxylation reaction, and, according to studies of the present inventors, there is a problematic decrease in the reactivity and a problematic hydrolysis of the phosphonitrile dichloride.
In the meantime, there is a known method for preparing phosphonitrilic acid ester in which the reaction solvent is not distilled off from the reaction solution that contains a phosphonitrile dichloride prepared from a phosphorus chloride and ammonium chloride and subjected to a reaction with an alcoholic compound and/or a phenolic compound as it is.
Moreover, methods for synthesizing a phosphonitrile dichloride used as main materials at the time of preparing a phosphonitrilic acid ester include (1) a method using phosphorus pentachloride, (2) a method using phosphorus trichloride, (3) a method using white phosphorus, and (4) a method using phosphorus nitride as a phosphorus source.
Extensive studies have been made with regard to the preparation method of a phosphonitrile dichloride for many years. As a typical technique, there is a known method in which phosphorus pentachloride and ammonium chloride are reacted in the presence of a multivalent metal compound catalyst, and the products containing a cyclic phosphonitrile dichloride oligomer is collected (for example, see JP-A-57-3705). In addition, there is a known method in which ammonia gas and hydrogen chloride gas are introduced into the reaction system to produce particulate ammonium chloride, which is then reacted with a phosphorus chloride to prepare a cyclic phosphonitrile dichloride (for example, see JP-A-49-47500). Furthermore, there is a known method in which a multivalent metal compound having Lewis acidity and a pyridine derivative such as quinoline are used as a catalyst and phosphorus pentachloride and ammonium chloride are reacted to selectively prepare a trimer (for example, see JP-A-62-39534).
From the prepared phosphonitrile dichloride, excessive ammonium chloride is removed usually by filtering the reaction slurry containing the phosphonitrile dichloride. Then, at least one operation step selected from the isolating steps described below are performed to isolate or purify the phosphonitrile dichloride from the reaction solution, and a product isolated or purified is used as a material of the following second step reaction, i.e., alkoxylation or aryloxylation reaction.    1) an operation in which the solvent is evaporated from the reaction solution and the crystal component deposited by condensing it (mainly composed of lower molecular weight cyclic compounds having m of 3 or 4 in the following general formula (12)) is separated by centrifugal separation, filtration, etc.;    2) an operation in which the solvent is evaporated from the reaction solution and linear and cyclic compounds are separated by adding a hydrocarbon solvent to the condensed or dried component;    3) an operation in which the reaction solution is contacted with water thereby extracting linear compounds in the aqueous phase;    4) an operation in which the content of the cyclic compounds having m of 3 or 4 in the following general formula (12) is increased by recrystalization purification or sublimation purification.
(wherein m represents an integer of 3 or more)
There are known examples of the above method in which the reaction solvent is not distilled off from the reaction solution that contains a phosphonitrile dichloride prepared from a phosphorus chloride, which phosphonitrile dichloride is reacted with an alcoholic compound and/or a phenolic compound. For example, in one method, monochlorobenzene is used as a reaction solvent and an alcohol and cyclic phosphonitrile dichloride are reacted in the presence of a pyridine derivative (for example, see U.S. Pat. No. 3,794,701). However, this method takes a long time for completing the reaction and the pyridine derivative used in a large amount is expensive and the recovery and the regeneration processes are cumbersome.
There is also a known method in which a linear phosphonitrile dichloride is prepared from the reaction of phosphorus pentachloride and ammonium chloride in a chlorine containing unsaturated hydrocarbon and an alcohol is made acted on the reaction solution thereby preparing a polyalkoxyphosphazene (for example, see Russian patent No. 385,980). In this method, straight chain unsaturated chlorinated hydrocarbons are the only described reaction solvent, but the industrial use may be a problem since some of these unsaturated chlorinated hydrocarbons are carcinogenic. Moreover, since the ammonium chloride does not contain an alkaline metal alcoholate but an alcohol is used at the time of the alkoxylation reaction of the phosphonitrile dichloride, the reactivity is very low, and it takes a long time for completing the reaction. Also in this technique, there is no description about the amount of moisture in the reaction system, and according to studies of the present inventors, there is a problem that moisture decreases the reactivity and the hydrolysis of the phosphonitrile dichloride.